The present invention relates to pneumatic systems for accurately measuring fluid pressures in order to remotely indicate the weight, volume, depth or other parameters associated with fluids in large processing or storage tanks or vessels. More particularly, the invention relates to pressure sensors for such pneumatic systems.
A pneumatic system of the type mentioned above is illustrated in U.S. Pat. No. 3,161,051, issued Dec. 15, 1954, the disclosure of which is incorporated herein by reference. Such a system generally includes a flush diaphragm-type sensing unit or transmitter mounted in the tank wall, near the bottom of the tank, with the tank side of the diaphragm being in contact with the fluid being measured. The opposite, or "indicator", side of the diaphragm is supplied with a substantially constant volume of air or other inert gas, which acts as an indicator fluid and is vented through a nozzle to an extent proportional to the position of the diaphragm. As a result, the pressure on the indicator side of the diaphragm is maintained at a level substantially equal to the pressure on the fluid or tank side of the diaphragm, and the pressure of the indicator fluid is constantly and dynamically proportional to the tank pressure, and hence the weight, volume or depth of the fluid in the tank. An example of an improved pneumatic system of the type described is disclosed in the copending U.S. patent application, Ser. No. 287,351, filed on July 29, 1981, and assigned to the same assignee as the invention described and claimed herein. The disclosure of such application is incorporated herein by reference.
Previous pneumatic systems have typically included soft rubber or other elastomeric diaphragm members stretched over the end of a sensor or transmitter assembly. However, such diaphram members lack the durability and strength for many applications that require a tough temperature-resistant, cleanable diaphragm member. Furthermore, such elastomeric diaphragm members have been found to be unsuitable for vacuum applications. As a result stainless steel diaphragm members have been used on sensor or transmitter assemblies in such applications. The prior stainless steel diaphragm members, however, have frequently been found to be inaccurate, with wide variations in pressure response. Also, because such prior stainless steel diaphragm members had to be secured to the sensor assembly by means of an annular clamp-type structure, they were not able to be mounted in a flush or substantially coplanar relationship with the inside wall of the tank. As a result, the prior sensor or transmitter assemblies have been found to introduce crevices or collection areas for the build-up and growth of bacteria, and therefore have been unsatisfactory for sanitary/aspetic applications. Furthermore, such protruding sensor or transmitter assemblies are difficult to clean, thus slowing down and adding to the expense of tank maintenance.
An improved sensor or transmitter assembly according to the present invention includes a thin stainless steel or other metallic diaphragm member that is retained in place without clamping members in an opening in the wall or other boundary of a tank or vessel. The diaphragm member is thus capable of a substantially or at least generally flush mounting configuration such that the opening is sealed substantially in the plane of the adjacent inside surface of the tank or vessel. The preferred diaphragm member is formed with a predetermined cross-sectional shape during assembly of the sensor or transmitter unit that ensures proper deflection and accurate pressure response. Also, the diaphragm-to-air nozzle relationship is uniformly calibrated during assembly of each sensor or transmitter unit thereby eliminating the need for system calibration when a new sensor unit is installed in an existing tank or vessel.
It is therefore an object of the present invention to provide an improved sensor or transmitter unit for pneumatic systems of the aforesaid type which has all the advantages of existing systems and yet overcomes the problems discussed above. A related object is the provision of such a sensor unit which is simple in construction, which does not materially increase operating costs, and which is readily adapted for retrofitting into existing systems with a minimum of difficulty.
These and other objects, features and advantages of the present invention will become apparent from the subsequent description and the appended claims, in conjunction with the accompanying drawings.